A conventional photodiode array device using photodiodes, for example, a planar photodiode array device 1 shown in FIG. 6 has an absorption layer (i-InGaAs) 1b and a cladding layer (i-InP) 1c both formed on one surface of a substrate (n+-InP) 1a. Four light-receiving regions 1d, which are p+ regions, are formed in the absorption layer 1b and the cladding layer 1c by diffusing Zn. Also, a plurality of anode rings 1e are formed on the cladding layer 1c. A cathode 1f, which is a thin Au—Ge layer, is formed over the other surface of the substrate 1a. In this photodiode array device 1, incident light is converted into electricity by the absorption layer 1b, and the resulting photocurrent is output from the anode ring 1e. 
In FIG. 6, hatching is omitted in order to avoid intricacy of lines, and this is the case with FIGS. 1, 3A to 3E and 5 referred to in the following description.